The present invention relates generally to gears and planetary gearsets and, in particular, to a planetary gearset with a multi-layer coated sun gear.
Gears, which are utilized to transmit torque from one rotating shaft to another for driving rotating equipment by utilizing engaging gear teeth, are well known. A plurality of interconnected gears is known in the art as a gearset. A typical planetary gearset includes a sun gear member, a plurality of planet gear members supported by a carrier member, and a ring gear member.
In automotive planetary gearsets, the sun gear member is usually connected to and rotated by a drive shaft. The outer periphery of the sun gear member includes a plurality of gear teeth formed thereon. Each of the sun gear teeth includes mating surfaces that engage with corresponding mating surfaces on respective gear teeth of a plurality of planet gears. The gear teeth on the planet gears also engage with gear teeth on the inner surface of a ring gear.
During the operation of automotive planetary gearsets, the sun gear is engaged with the planet gears and transmits high levels of torque. As a result, the sun gear teeth are exposed to heavy periodic loads. The number of times the load is applied to each sun gear tooth during a single revolution of the sun gear is proportional to the number of planet gears in the planetary gearset. This repeated loading often results in fatigue of the mating surfaces of the sun gear teeth.
In a typical automotive transmission utilizing a planetary gearset, the sun gear is not rigidly mounted on the drive shaft, but is connected via a set of internal splines that engage with a corresponding set of external splines on the drive shaft. The splines allow the sun gear to “float” i.e., to make small random movements, in a radial direction and still fully transmit the torque. These radial movements help to re-distribute the torque evenly among the planet gears, increasing the torque capacity of the gearset overall. Because of this floating connection, the sun and planet gears occasionally experience dynamic loads in addition to the transmitted torque load.
A typical process used for manufacturing automotive gears includes hobbing and shaving, followed by a thermo-chemical treatment, such as carburization. As a result of these manufacturing processes, the mating surfaces of the gear teeth are covered with asperities or small projections extending upward therefrom. Typically, the as-machined surface roughness of the sun and planet gears reach the micron level, with the Ra parameter normally being between 0.20 to 0.50 microns. During gearset operation, high stresses may develop at the tips of the asperities.
Also, the shape and dimensions of the gears overall and the shape and dimensions of the mating surfaces of the gear teeth may deviate from the desired and prescribed shape by up to 20 microns as a result of small machining errors. This may lead to areas of high contact stress.
To improve tribological conditions during operation of the gearset, the mating surfaces of the gear teeth are supplied with lubricating oil. Due to hydrodynamic effects, a thin oil film usually forms between moving parts, such as mating surfaces of gear teeth. When the surface asperity heights are equal to or greater than the oil film thickness, the oil film is punctured and metal-to-metal contact occurs. This is known in the art as boundary lubrication. Boundary lubrication may result in localized micro-welding, an increase in surface shear stress, and surface fatigue. This is often observed in uncoated gears during the run-in or break-in period. When the surface asperity heights become smaller than the hydrodynamic oil film, metal-to-metal contact ceases, the coefficient of friction drops and the fatigue processes are either arrested altogether or significantly slowed. This type of lubrication is known in the art as elasto-hydrodynamic (EHD) lubrication.
Furthermore, in a planetary gearset, the planet gears are supported by a planet carrier, which affects the interaction of the planet gears with the sun and the ring gears. Due to manufacturing errors and non-compensated elastic deflections of all the interconnected parts, including the sun gear, the planet gears, the ring gear, the carrier, and the shafts, misalignment of mating surfaces is inevitable, resulting in high contact stresses on the mating surfaces. The local stress between mating surfaces of two misaligned gears may reach the level of the gear hardness, which is typically in the range of 8 GPa. This stress can result in an undue fatigue of the planetary gearset, premature failure of the gears and damage to the driven or driving equipment.
It is desirable, therefore, to reduce the level of stress acting on the mating surfaces of the gear teeth of a planetary gearset. It is also desirable to provide a planetary gearset with a sun gear that will extend the life of the planetary gearset and associated driven equipment. It is also desirable to decrease the height of the asperities. Since the oil film thickness remains unchanged, this will change the lubrication regime from boundary lubrication to full-film EHD lubrication. In addition, it is desirable to produce gearsets with reduced manufacturing errors and lower deviation from the desired shape.